E7 Pulmonary gas exchange

Topic

Gas exchange is the process by which oxygen and carbon dioxide cross the membrane separating the alveoli from the pulmonary capillaries via passive diffusion. The rate of diffusion increases with the partial pressure difference of the gas and the available surface area for exchange, and decreases as the thickness of the alveolocapillary membrane increases. Consequently, a reduction in surface area or a thickening of this barrier hinders exchange.

Carbon dioxide is far more soluble than oxygen and diffuses approximately twenty times faster. When a disease alters the alveolocapillary membrane, oxygen exchange is usually affected first, leading to hypoxemia, whereas hypercapnia generally appears when the impairment is more severe or when there are additional ventilation abnormalities.

Once in the bloodstream, most oxygen binds to hemoglobin in red blood cells. Its four subunits exhibit cooperativity: the binding of one oxygen molecule facilitates the binding of subsequent ones. This produces a sigmoid curve representing the relationship between oxygen partial pressure and hemoglobin saturation, with a point near 27 mmHg where approximately half of the binding capacity is occupied.

Hemoglobin's affinity changes according to tissue conditions. Increased acidity, carbon dioxide levels, or temperature reduce its affinity and facilitate oxygen release. Since these conditions arise precisely in metabolically active tissues, hemoglobin releases more oxygen where the demand is greatest.

The amount of oxygen initially available in the alveoli depends on the inspired oxygen fraction, barometric pressure, and alveolar carbon dioxide concentration. At high altitudes, the drop in barometric pressure reduces alveolar oxygen partial pressure and can cause hypoxia. Comparing the calculated alveolar oxygen pressure with the measured arterial blood pressure yields the alveolo-arterial gradient. When this value rises, hypoxemia cannot be explained solely by reduced availability of inspired oxygen; instead, it indicates an impairment of gas exchange within the lung.

One mechanism causing this impairment is shunting, where a portion of blood reaches the arterial circulation without coming into contact with ventilated alveoli—such as when alveoli are collapsed or fluid-filled. Because this blood does not contact alveolar oxygen, the hypoxemia shows little response to supplemental oxygen. The opposite phenomenon is alveolar dead space: ventilated alveoli receive little or no perfusion, which can occur when a clot blocks circulation to them. This ventilation does not contribute effectively to gas exchange. Between these two extremes lies ventilation-perfusion mismatch, a very common cause of impaired gas exchange.

Carbon dioxide also plays a direct role in regulating acid-base balance. Blood acidity depends fundamentally on the ratio between CO₂—regulated rapidly by the lungs—and bicarbonate, which is controlled more slowly by the kidneys. The lungs can alter CO₂ elimination within minutes by adjusting ventilation, whereas the kidneys require hours or days to modify bicarbonate retention or excretion.

Acid-base disorders arise when there is a primary change in one of these components. Excessive CO₂ retention or elimination leads to respiratory disorders, while primary changes in bicarbonate result in metabolic disorders. The system not initially affected attempts to compensate for the change: the kidneys modify bicarbonate levels in response to respiratory disorders, while ventilation alters CO₂ levels in response to metabolic disorders.

Taken together, alveolar-capillary diffusion, oxygen transport by hemoglobin, alveolar oxygen availability, the ventilation-perfusion ratio, and the coordinated regulation of CO₂ and bicarbonate allow for the interpretation of arterial blood gases. Hypoxemia and hypercapnia provide information regarding respiratory efficiency; The alveolar-arterial gradient helps localize abnormalities within the lung; shunt and dead space identify different forms of inefficiency, and the combination of CO₂ and bicarbonate allows for the recognition of major acid-base disorders and their compensatory responses. Thus, arterial blood gas analysis integrates pulmonary gas exchange and the body's acid-base balance into a single assessment.

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